The invention relates to a central solar receiver with a volumetric radiation absorber of the kind that absorbs concentrated solar radiation at a high temperature and transfers heat to a working fluid and, particularly, to such a receiver where the absorber and the working fluid constitute a multicomponent fluid mixture so that the working fluid components are in direct contact with the radiation absorbing components. Thereby, the absorbing components of the mixture either heat up the working fluid components or participate in a chemical reaction therewith or act as catalysts for a chemical reaction among the other components.
The phases of absorbing components in the multicomponent fluid mixture may be different from that of the working fluid components thereof. Thus, often the mixture consists of solid particles suspended in the working fluid. For example, Evans et al. (J. Solar Energy Eng. 109, pp. 134-142, 1987) discloses circulation of a mixture of gas and solid particles through a large atmospheric pressure cavity, which is isolated from the environment by a forced gas flow across the aperture. Rightley et al. (Solar Energy, 48, pp. 363-374, 1992) suggests a free-falling particle receiver, where a curtain of spherical beads was exposed to solar radiation through an open aperture in the receiver. Imhof (Solar Energy Materials, 24, pp. 733-741, 1991) and Steinfeld et al. (J. Solar Energy Eng., 114, pp. 171-174, 1992) investigate an atmospheric cyclone reactor, where a vortex-type flow provides for separation of solid particles from the carrier fluid. A double walled cylindrical cavity with swirling flow and windowless aperture is disclosed by Ganz et al. (7th Int. Symp. Solar Thermal Concentrating Technologies, Moscow, Russia, 1994). A rotary kiln configuration is employed by Kelbert and Royere (Proc., 4th Int. Symp. Solar Thermal Technology, pp. 327-336, Santa-Fe, N.Mex., 1988) to heat sand to 1200 K. In all these designs the working fluid is introduced into the receiver at atmospheric pressure with the radiation entering the receiver through the open aperture.
With the solar receivers having a volumetric radiation absorber, the thermal efficiency of the absorber depends among others on the rate of heat transfer from the radiation absorber to the working fluid. A high rate of heat transfer requires a high mass flow rate of the working fluid. To increase such mass flow rate while minimizing friction losses, the working fluid in a central solar receiver with a volumetric radiation absorber is pressurized. Elevated pressure of heated working fluid withdrawn from a central solar receiver is also required for applications, such as driving an electric power generating turbine. To reconcile the need for a pressurized working fluid with the requirement for direct heating of the volumetric absorber, the absorber and working fluid have to be contained within a sealed enclosure having a window that is transparent to the incident concentrated radiation.
In addition, sealing off of the volumetric radiation absorber in a central solar receiver with the provision of a transparent window is, as a rule, also required where the working fluid is not pressurized, in order to prevent the working fluid and any products of reaction between its components from escaping into the ambient atmosphere.
A central solar receiver satisfying the above requirements is disclosed, for example, in U.S. Pat. No. 5,323,764. However, this design is suitable only for the volumetric radiation absorber in the form of a stationary solid body such as an array of absorber members.
Many attempts have been made to design a windowed receiver with a particulate absorber. Thus, Miller (Proc. 6th Int. Symp. Solar Thermal Concentrating Technologies, pp. 371-385, Almeria, Spain, 1992) proposes a concept for a vertical cylindrical central receiver, which may be windowed; this configuration cannot support high pressure. Gregg et al. (Solar Energy, 25, pp. 353-364, 1980) gasifies fixed beds of coal, biomass and oil shales in an L-shaped reactor, with sunlight penetrating through a quartz window. Although the window is kept clean and cool by gas injection, this design allows only low pressure, and is difficult to integrate with continuous processes. Gasification of oil shales by pyrolysis in a bed fluidized by pressurized argon is disclosed in Ingel et al. (Energy, 18, pp. 827-842, 1993 and Energy17, pp. 1189-1197, 1993), but the thermal efficiency in this configuration is low and cannot support high pressure. Antal et al. (Solar Energy, 30, pp. 299-312, 1983) attempted flash pyrolysis of biomass in a vertical quartz tube. The tube was damaged by devitrification where carbon had deposited. Development of a windowed circulating fluidized bed receiver (Koenigsdorff, Proc. 6th Int. Symp. Solar Thermal Concentrating Technologies, pp. 347-358, Almeria, Spain, 1992, and Litterst, Proc., 6th Int. Symp. Solar Thermal Concentrating Technologies, pp. 359-369, Almeria, Spain, 1992) was not successful since in this case, similarly to all the designs described above, no solution was found to prevent contact between the particles of the absorbing component and the window of the receiver, causing mechanical and thermal damage and deterioration of optical properties thereof. Moreover, none of the proposed design configurations could not support high pressure.